Control system using a phase modulation capable acousto-optic modulator for diverting laser output intensity noise to a first order laser light beam and related methods

ABSTRACT

A laser system may include a laser source configured to generate a laser light beam and an acousto-optic modulator (AOM). The AOM may include an acousto-optic medium configured to receive the laser light beam, and a phased array transducer comprising a plurality of electrodes coupled to the acousto-optic medium and configured to cause the acousto-optic medium to output a zero order laser light beam and a first order diffracted laser light beam. The system may further include a beamsplitter downstream from the AOM and configured to split a sampled laser light beam from the zero order laser light beam, a photodetector configured to receive the sampled laser light beam and generate a feedback signal associated therewith, and a radio frequency (RF) driver configured to generate an RF drive signal to the phased array transducer electrodes so that noise is diverted to the first order diffracted laser light beam based upon the feedback signal.

TECHNICAL FIELD

The present invention relates to the field of optical devices, and, moreparticularly, to acousto-optic modulators for lasers and relatedmethods.

BACKGROUND

Acousto-optic modulators, sometimes referred to as Bragg cells, diffractand shift light using sound waves at radio frequency. These devices areoften used for Q-switching, signal modulation in telecommunicationssystems, laser scanning and beam intensity control, frequency shifting,and wavelength filtering in spectroscopy systems. Many otherapplications lend themselves to using acousto-optic devices.

In such acousto-optic devices, a piezoelectric transducer, sometimesalso referred to as an RF transducer, is secured to an acousto-opticbulk medium as a transparent optical material, for example, fusedsilica, quartz or similar glass material. An electric RF signaloscillates and drives the transducer to vibrate and create sound waveswithin the transparent medium which effect the properties of an opticalfield in the medium via the photo elastic effect, in which a modulatingstrain field of an ultrasonic wave is coupled to an index of refractionfor the acousto-optic bulk medium. As a result, the refractive indexchange in amplitude is proportional to that of sound.

The index of refraction is changed by moving periodic planes ofexpansion and compression in the acousto-optic bulk material. Incominglight scatters because of the resulting periodic index modulation andinterference, similar to Bragg diffraction.

Acousto-optic modulators are preferred in many applications because theyare faster than tiltable mirrors and other mechanical devices. The timeit takes for the acousto-optic modulator to shift an exiting opticalbeam is limited to the transit time of the sound wave. The acousto-opticmodulators are often used in Q-switches where a laser produces a pulsedoutput beam at high peak power, typically in the Kilowatt range. Thisoutput could be higher than lasers operating a continuous wave (CW) orconstant output mode.

Examples of acousto-optic modulator devices and similar acousto-opticsystems are disclosed in commonly assigned U.S. Pat. Nos. 4,256,362;5,923,460; 6,320,989; 6,487,324; 6,538,690; 6,765,709; and 6,870,658,the disclosures of which are hereby incorporated by reference in theirentireties.

Some applications using acousto-optic devices modulate the intensity ofan optical beam. This modulation may create small deviations in theoutput angle of the diffracted beam because of the local thermaltransients introduced when the RF modulation waveform to the device isturned ON and OFF. These thermal transients may negatively impact theresolution and location of the focused spot, which may be produced. Oneadvantageous approach which may be used to help enhance the resolutionof acousto-optic devices is set forth in U.S. Pat. No. 7,538,929 toWasilousky, which is assigned to the present Applicant and is herebyincorporated herein in its entirety by reference. Wasilousky disclosesan acousto-optic modulator which includes an acousto-optic bulk mediumand transducer attached to the acousto-optic bulk medium and formed as alinear array of electrodes. A transducer driver is connected to eachelectrode and is coherently phase driven to alter the angular momentumdistribution of an acoustic field and alternately allow and inhibitphase matching between the optical and acoustic field and produce adesired intensity modulation of an optical wavefront.

Despite the existence of such configurations, further advancements inlaser systems using acousto-optic modulators may be desirable in certainapplications.

SUMMARY

A laser system may include a laser source configured to generate a laserlight beam and an acousto-optic modulator (AOM). The AOM may include anacousto-optic medium configured to receive the laser light beam, and aphased array transducer comprising a plurality of electrodes coupled tothe acousto-optic medium and configured to cause the acousto-opticmedium to output a zero order laser light beam and a first orderdiffracted laser light beam. The system may further include abeamsplitter downstream from the AOM and configured to split a sampledlaser light beam from the zero order laser light beam, a photodetectorconfigured to receive the sampled laser light beam and generate afeedback signal associated therewith, and a radio frequency (RF) driverconfigured to generate an RF drive signal to the phased array transducerelectrodes so that noise is diverted to the first order diffracted laserlight beam based upon the feedback signal.

More particularly, the RF driver may be configured to drive alternatingelectrodes of the phased array transducer electrodes with differentphases. By way of example, the RF driver may be configured to drive thealternating electrodes with different phases within a range of 0° to180°. Furthermore, the RF power level associated with the drive signalmay have a constant power.

The sampled laser light beam may utilize ≤3% of the light of the zeroorder laser light beam, for example. Also by way of example, at the setpoint the first order diffracted light beam may also utilize a similarlysmall percentage of the laser light from the laser source. For example,the first order diffracted laser light beam have ≤3% of the light fromthe laser source.

In one example embodiment, the laser system may further include an iontrap, and the beamsplitter may be configured to direct the zero orderlaser light beam from the AOM to the ion trap. In accordance withanother example, the laser system may further include a semiconductorworkpiece having a photoresist layer, and the beamsplitter may beconfigured to direct the zero order laser light beam from the AOM to thephotoresist layer. In still another example, the laser system mayfurther include a micromachining workpiece, and the beamsplitter may beconfigured to direct the zero order laser light beam from the AOM to themicromachining workpiece.

A related method may include generating a laser light beam using a lasersource and directed at an acousto-optic medium, causing theacousto-optic medium to output a zero order laser light beam and a firstorder diffracted laser light beam using a phased array comprising aplurality of electrodes coupled to the acousto-optic medium, andsplitting a sampled laser light beam from the zero order laser lightbeam using a beamsplitter downstream from the acousto-optic medium. Themethod may further include generating a feedback signal associated withthe sampled laser light beam using a photodetector, and generating an RFdrive signal to the phased array transducer electrodes with an RF driverso that noise is diverted to the first order diffracted laser light beambased upon the feedback signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a laser system including aphase-capable acousto-optic modulator (AOM) in accordance with anexample embodiment.

FIGS. 2 and 3 are schematic circuit diagrams illustrating differentelectrode connection configurations and associated driving signalstherefor which may be used with the systems of FIGS. 1-3.

FIG. 4 is a flow diagram illustrating method aspects associated with thesystem of FIG. 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present description is made with reference to the accompanyingdrawings, in which exemplary embodiments are shown. However, manydifferent embodiments may be used, and thus the description should notbe construed as limited to the particular embodiments set forth herein.Rather, these embodiments are provided so that this disclosure will bethorough and complete. Like numbers refer to like elements throughout.

By way of background, excessive noise levels from laser sources inoptical illumination systems generate instabilities and errors. Inparticular, systems that manipulate the quantum states of particles,atoms and electrons, typically require extreme stability. Beam pointingerrors correlate to noise in quantum state manipulation systems.Moreover, beam pointing stability due to thermal transients in the bulkmaterial of active acousto-optic devices in an optical illuminationsystem affect many applications, but especially those designed forquantum state illumination.

Referring initially to FIGS. 1 and 4, a laser system 30 and associatedmethod aspects which may provide enhanced stability and noise reductionare first described. Beginning at Block 61 of the flow diagram 60, thelaser system 30 illustratively includes a laser source 31 configured togenerate a laser light beam, at Block 62. In accordance with one exampleembodiment, a Paladin Advanced 355 nm mode locked UV laser source fromCoherent, Inc. of Santa Clara, Calif. may be used, although othersuitable laser sources may also be used in different embodiments. Thesystem 30 further illustratively includes an acousto-optic modulator(AOM) 32. The AOM illustratively includes an acousto-optic medium 33configured to receive the laser light beam from the laser source 31, anda phased array of electrodes 34 coupled to the acousto-optic medium. Theacousto-optic medium 33 may include a piezoelectric transducer and bulkacousto-optic medium (e.g., silica, quartz, glass, etc.), as discussedabove. The phased array of electrodes 34 are configured to cause theacousto-optic medium 33 to output a zero order laser light beam to anoptical target 38, and a first order diffracted laser light beam, atBlock 63, as will be discussed further below.

The system further illustratively includes a beamsplitter 35 downstreamfrom the AOM 32 which is configured to split a sampled laser light beamfrom the zero order laser light beam, at Block 64. The beamsplitter 35need only divert a small portion of light from the zero order laserlight beam into the sampled laser light beam (e.g., ≤3%) to provideadequate feedback to a radio frequency (RF) driver 36 for driving thephase array of electrodes 34. More particularly, a photodetector 37 isconfigured to receive the sampled laser light beam and generate anelectrical feedback signal for the RF driver 36 based upon the sampledlaser light beam. As such, the RF driver 36 is able to generate one ormore RF drive signals to the phased array of electrodes 34 to generatethe zero order beam and the first order diffracted beam accordingly,which illustratively concludes the method of FIG. 4 (Block 67).

In particular, the RF driver 36 drives the phased array of electrodes 34such that noise measured from the feedback signal is diverted to thefirst order diffracted laser light beam, which may be directed to a beamdump 39 (or simply away from the optical target 38). This advantageouslyprovides noise cancelation by diffracting a relatively small amount oflight from the zero order beam (e.g., ≤3%) into the first orderdiffracted beam by changing the phase of the RF drive signal toalternating electrode elements of the phased array of electrodes 34. Inparticular, the feedback signal is inverted and sent to the phasemodulation capable AOM 32 to subtract and correct for the inherent noisein the laser.

This may be done while the RF power applied to the acousto-optic medium33 remains essentially constant which helps to eliminate beam pointingerrors which may otherwise be associated with varying thermal transientsdue to changing RF power levels, as may be experienced with typicalamplitude modulation AOMs, for example. Stated alternatively, by onlyeffecting the phase of the RF drive signal to the N element phased arrayelectrode pattern on the AOM and leaving the RF power level essentiallyconstant, this advantageously reduces the laser intensity noiseappearing on the zero order beam while still retaining a positionallystable beam.

More particularly, referring additionally to FIGS. 2 and 3, two exampleconfigurations for driving alternating electrodes 40 of the phased arrayof electrodes 34 with different phases to provide the zero and firstorder beam configuration described above are now described. In the firstconfiguration (FIG. 2), the first and third driving signals (shown onthe right hand side of FIG. 2) provided to corresponding odd numberedelectrodes are 180° out of phase with the second and fourth drivingsignals provided to corresponding even numbered electrodes. In thesecond configuration (FIG. 3), first and second drive signals arerespectively connected to odd and even electrodes in an interdigitatedfashion as shown, and as before these drive signals are 180° out ofphase to one another. In this way, directly adjacent electrodes aredriven at opposite phases to one another. However, it should be notedthat the RF drive signals need not always be 180° out of phase, i.e.,they may be somewhere between 0° and 180° to vary the level of phasematching occurring in the AO diffraction process, thereby selectivelyaltering the amount of light directed from the zero order beam into thefirst order beam.

The system 30 accordingly combines intensity modulation via RF-phasevariation on a phased array transducer with active optical feedback toaccomplish noise cancelation in an optical illumination system.Moreover, performing phase modulation by flipping the phase ofalternating elements of a multi-element phased array has inherentlybetter pointing stability because the RF power applied to the deviceremains essentially constant, as noted above. Further, applying this tothe zero order beam allows the RF power to remain low, reducing thepotential of thermal gradients and thermal transients.

The system 30 may accordingly provide advantages with respect tonumerous different types of optical targets. By way of example, in oneconfiguration the optical target 38 may be an ion trap, such as in aquantum computing device. In accordance with another example, theoptical target 38 may be a semiconductor workpiece to performphotolithographic patterning of a photoresist layer, for example. Instill another example, the optical target 38 may be a micromachiningworkpiece. It should be noted that the laser system 30 may be used withother optical targets in different embodiments as well.

Other example systems in which the above-described stability and noisereduction techniques may be used are set forth in the followingco-pending applications: attorney docket no. GCSD-2899 (62084) entitledMULTI-CHANNEL LASER SYSTEM INCLUDING AN ACOUSTO-OPTIC MODULATOR (AOM)AND RELATED METHODS; and attorney docket no. GCSD-2900 (62087) entitledMULTI-CHANNEL ACOUSTO-OPTIC MODULATOR (AOM) AND RELATED METHODS. Both ofthese applications are assigned to the present Applicant HarrisCorporation and are hereby incorporated herein in their entireties byreference.

Many modifications and other embodiments will come to the mind of oneskilled in the art having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it isunderstood that the disclosure is not to be limited to the specificembodiments disclosed, and that modifications and embodiments areintended to be included within the scope of the appended claims.

1. A laser system comprising: a laser source configured to generate alaser light beam; an acousto-optic modulator (AOM) comprising anacousto-optic medium configured to receive the laser light beam, and aphased array transducer comprising a plurality of electrodes coupled tothe acousto-optic medium and configured to cause the acousto-opticmedium to output a zero order laser light beam and a first orderdiffracted laser light beam; a beamsplitter downstream from the AOM andconfigured to split a sampled laser light beam from the zero order laserlight beam; a photodetector configured to receive the sampled laserlight beam and generate a feedback signal associated therewith; and aradio frequency (RF) driver configured to generate an RF drive signal tothe phased array transducer electrodes so that noise is diverted to thefirst order diffracted laser light beam based upon the feedback signal.2. The laser system of claim 1 wherein the RF driver is configured todrive alternating electrodes of the phased array transducer electrodeswith different phases.
 3. The laser system of claim 2 wherein the RFdriver is configured to drive the alternating electrodes with differentphases within a range of 0° to 180°.
 4. The laser system of claim 1wherein an RF power level associated with the RF drive signal has aconstant power.
 5. The laser system of claim 1 wherein the sampled laserlight beam utilizes ≤3% of the light of the zero order laser light beam.6. The laser system of claim 1 wherein the first order diffracted laserlight beam has ≤3% of the light from the laser source.
 7. The lasersystem of claim 1 further comprising an ion trap, and wherein thebeamsplitter is configured to direct the zero order laser light beamfrom the AOM to the ion trap.
 8. The laser system of claim 1 furthercomprising a semiconductor workpiece having a photoresist layer, andwherein the beamsplitter is configured to direct the zero order laserlight beam from the AOM to the photoresist layer.
 9. The laser system ofclaim 1 further comprising a micromachining workpiece, and wherein thebeamsplitter is configured to direct the zero order laser light beamfrom the AOM to the micromachining workpiece.
 10. A laser systemcomprising: a laser source configured to generate a laser light beam; anacousto-optic modulator (AOM) comprising an acousto-optic mediumconfigured to receive the laser light beam, and a phased arraytransducer comprising a plurality of electrodes coupled to theacousto-optic medium and configured to cause the acousto-optic medium tooutput a zero order laser light beam and a first order diffracted laserlight beam; a beamsplitter downstream from the AOM and configured tosplit a sampled laser light beam from the zero order laser light beam; aphotodetector configured to receive the sampled laser light beam andgenerate a feedback signal associated therewith; and a radio frequency(RF) driver configured to generate an RF drive signal to the phasedarray transducer electrodes so that noise is diverted to the first orderdiffracted laser light beam based upon the feedback signal, the RFdriver driving alternating electrodes of the phased array transducerelectrodes with different phases, and an RF power level associated withthe RF drive signal having a constant power.
 11. The laser system ofclaim 10 wherein the RF driver is configured to drive the alternatingelectrodes with different phases within a range of 0° to 180°.
 12. Thelaser system of claim 10 wherein the sampled laser light beam utilizes≤3% of the light of the zero order laser light beam.
 13. The lasersystem of claim 10 wherein the first order diffracted laser light beamhas ≤3% of the light from the laser source.
 14. The laser system ofclaim 10 further comprising an ion trap, and wherein the beamsplitter isconfigured to direct the zero order laser light beam from the AOM to theion trap.
 15. The laser system of claim 10 further comprising asemiconductor workpiece having a photoresist layer, and wherein thebeamsplitter is configured to direct the zero order laser light beamfrom the AOM to the photoresist layer.
 16. The laser system of claim 10further comprising a micromachining workpiece, and wherein thebeamsplitter is configured to direct the zero order laser light beamfrom the AOM to the micromachining workpiece.
 17. A method comprising:generating a laser light beam using a laser source directed at anacousto-optic medium; causing the acousto-optic medium to output a zeroorder laser light beam and a first order diffracted laser light beamusing a phased array transducer comprising a plurality of electrodescoupled to the acousto-optic medium; splitting a sampled laser lightbeam from the zero order laser light beam using a beamsplitterdownstream from the acousto-optic medium; generating a feedback signalassociated with the sampled laser light beam; and generating a radiofrequency (RF) drive signal to the phased array transducer electrodeswith an RF driver so that noise is diverted to the first orderdiffracted laser light beam based upon the feedback signal.
 18. Themethod of claim 17 wherein generating the RF drive signal comprisesgenerating the RF drive signal to drive alternating electrodes of thephased array transducer electrodes with different phases.
 19. The methodof claim 17 wherein an RF power level associated with the RF drivesignal has a constant power.
 20. The method of claim 17 wherein thesampled laser light beam utilizes ≤3% of the light of the zero orderlaser light beam.
 21. The method of claim 17 wherein the first orderdiffracted laser light beam has ≤3% of the light from the laser source.22. The method of claim 17 wherein splitting the beam further comprisesdirecting the zero order laser light beam to an ion trap.
 23. The methodof claim 17 wherein splitting the beam further comprises directing thezero order laser light beam to a photoresist layer on a semiconductorworkpiece.
 24. The method of claim 17 wherein splitting the beam furthercomprises directing the zero order laser light beam to a micromachiningworkpiece.